The present invention relates to radar systems and methods generally, and more specifically to methods of detecting a jammer.
U.S. Pat. No. 5,600,326 to Yu, et al. describes a system for adaptive beamforming so as to null one mainlobe and multiple sidelobe jammers. Yu addresses a problem wherein the monopulse technique for direction of arrival (DOA) estimation failed when there was sidelobe jamming (SLJ) and/or main lobe jamming (MLJ). If not effectively countered, electronic jamming prevents successful radar target detection and tracking. The situation is exacerbated by introduction of stealth technology to reduce the radar cross section (RCS) of unfriendly aircraft targets. The frequency dependence of the RCS encourages use of lower microwave frequency bands for detection. This leads to large apertures to achieve angular resolution. Large apertures to achieve small beamwidth results in interception of more jamming. On the other hand, constrained apertures lead to wider beamwidth, which implies interception of more mainlobe jamming.
Adaptive beamforming techniques have been used to form a beam having one or more nulls pointing in the direction of one or more respective jammers. When there is no jamming, Taylor and Bayliss weightings are typically used for sum beams and difference beams, respectively, so as to have a narrow mainlobe and low sidelobes. The quiescent Taylor and Bayliss weightings are designed for reducing the sidelobes in a practical system. In the presence of jamming, the weights are adapted so as to form nulls responsive to the jammers.
Adaptive receiving arrays for radar maximize the ratio of antenna gain in a specified scan direction to the total noise in the output signal. The sum and difference beams at array outputs are determined by adaptive receiving array techniques, which serve to null the interference sources. The adaptivity involves using multipliers to apply an adaptive weight to antenna array signals furnished at multiplier inputs. Because of the adaptivity, the sum and difference patterns vary with the external noise field and are distorted relative to the conventional monopulse sum and difference beams, which possess even and odd symmetry, respectively, about a prescribed boresight angle. The adaptive weights for the sum and difference beams are determined so that the antenna patterns are distorted. This technique cancels both the mainlobe and sidelobe jammers but distorts the monopulse ratio.
Yu et al. describe a sample matrix inverse approach for jamming cancellation, which effectively forms nulls responsive to jammers. The covariance matrix is inverted in order to form the adaptive weighting coefficients. If one of the jammers is within the mainbeam, a null is formed responsive to the mainlobe jammer and the mainbeam is distorted. In order to maintain the mainbeam without distortion, the mainlobe jammer effect is excluded from the covariance matrix estimate. This may be accomplished by using a modified covariance matrix in forming the adapted row beamforming weights, from which information of the mainlobe jammer has been removed (so there is no null responsive to the mainlobe jammer). Yu et al. use prefiltering to block the mainlobe jammer.
Although the matrix inverse approach can generate desired adaptive weights for pointing nulls toward jammers, this technique does not output the locations of the jammers. To implement active countermeasures (such as sending energy at a particular frequency or band of frequencies to a jammer), it is necessary to know the location of the jammer. In order to determine the DOA of a jammer using the prior art techniques, it was necessary to xe2x80x9cpointxe2x80x9d the receiving antenna array at the jammer, essentially placing the jammer in the mainlobe. Thus, an improved system is desired.
The present invention is a method and system for locating a radar jammer. Sampled aperture data are received from an antenna array. The sampled aperture data include data that do not correspond to echo returns from a beam transmitted by the antenna. A covariance matrix is generating using the sampled aperture data. An eigenvalue decomposition is performed on the covariance matrix. A direction of arrival is determined from which at least one jammer is transmitting a signal included in the sampled aperture data, based on the eigenvalue decomposition.